Current classification of the genus Alphavirus includes 29 species that can be classified into nine complexes based on antigenic and/or genetic similarities (Powers et al., J Virol., 2001, 75(21):10118-31). Barmah Forest, Ndumu, Middelburg, and Semliki Forest complexes consist of almost exclusively Old World viruses whereas Venezuelan equine encephalitis (VEE), Eastern equine encephalitis (EEE), and Trocara complexes are comprised of New World viruses (id.). Western equine encephalitis (WEE) complex contains both Old World (Whataroa and Sindbis) and New World (Aura) viruses as well as recombinant viruses (WEE, Highland J, Fort Morgan, and Buggy Creek) (Powers et al., J Virol., 2001, 75(21):10118-31; Hahn et al., Proc Natl Acad Sci USA. 1988, 85(16):5997-6001; Weaver et al., J. Virol. 1997, 71:613-623). The latter viruses are decedents of a recombinant virus that obtained nonstructural and capsid genes from an EEE-like virus and the remaining genes from a Sindbis-like virus (Hahn et al., Proc Natl Acad Sci USA. 1988, 85(16):5997-6001; Weaver et al., J. Virol. 1997, 71:613-623). Lastly, the aquatic alphavirus, salmonid alphavirus (SAV), consists of two species, salmon pancreatic disease virus (SPDV) and sleeping sickness virus (SDV) that are distantly related to all other alphaviruses (Weston et al., J Virol. 2002, 76(12):6155-63).
Most alphaviruses are maintained in natural cycles between arthropod vectors, mainly mosquitoes, and susceptible vertebrate hosts (Strauss and Strauss, Microbiol Rev. 1994, 58(3):491-562). Occasionally, these cycles can spill over into the human and animal populations, and can cause disease. Human infections with Old World viruses such as Ross River (RRV), chikungunya (CHIKV), and Sindbis (SINV) are characterized by febrile illness, rash and polyarthritis (id.). In contrast, infections with New World viruses, Venezuelan equine encephalitis (VEEV), Eastern equine encephalitis (EEEV) and Western equine encephalitis (WEEV), can cause fatal encephalitis. The ability of alphaviruses to infect both invertebrates and vertebrates facilitates a broad host range that enables the viruses to be maintained in ecological niches with sporadic outbreaks in humans and animals. As such, alphaviruses have been shown to either naturally or experimentally infect many vertebrate and invertebrate hosts. Alphaviruses have been shown to infect mosquito species encompassing three genera (Aedes sp., Culex sp., Anopheles sp.) as well as ticks and lice (Griffin. Alphaviruses, In: Fields B N, Knipe D M, Howley P M, editors. Virology. 5th edition. New York, N.Y.: Lippincott-Raven; Pages 1023-68; Linthicum et al., J Med Entomol. 1991, 28(3):405-9; La Linn et al., J Virol. 2001, 75(9):4103-9). Vertebrate hosts include fish, equids, birds, amphibians, reptiles, rodents, pigs, humans, and non-human primates (Griffin. Alphaviruses, In: Fields B N, Knipe D M, Howley P M, editors. Virology. 5th edition. New York, N.Y.: Lippincott-Raven; Pages 1023-68; Burton et al., Science 1966, 154(752):1029-31). Consequently, they can be readily cultured in vitro in many vertebrate and invertebrate cell lines (Way et al., J Gen Virol. 1976, 30(1):123-30; Sarver and Stollar, Virology 1977, 80(2):390-400; Igarashi, J Gen Virol. 1978, 40(3):531-44). Whereas distantly related fish alphaviruses, which are not known to have arthropod vectors, exhibit a narrow host range that is at least partially due to temperature (Weston et al., Virology 1999, 256(2):188-95; Villoing et al., J Virol. 2000, 74(1):173-83; Graham et al., J Fish Dis. 2008, 31(11):859-68).
The viral factor(s) that underlie the broad host range of mosquito-borne alphaviruses are poorly understood. Host-restricted viruses may provide insight into these factor(s) and provide vector delivery platforms for expression or attenuation in specific hosts. But until the present invention, no mosquito-only alphaviruses were known in the genus Alphavirus. 